Measurement of static friction coefficient between flat surfaces

An experimental device, called the centrifugal friction apparatus (CFA), was developed which is capable of measuring the static coefficient of friction, mu, between flat surfaces with very low normal forces. Experiments were performed with the CFA using the polished surface of n-type arsenic doped silicon wafers to investigate static frictional behavior under differing light load conditions. For loads less than 100 mgf, the average mu and the standard deviation were found to increase as the normal load decreased. For loads between 100 mgf and 1.1 gf mu was found to be a constant in the range of 0.30-0.40. Results suggest that despite extreme smoothness the real area of contact is still significantly smaller than the apparent area of contact and mu is still a function of load. Tests with solid particles between the silicon mating surfaces showed that under low load conditions (<0.9 Pa), the change in mu was negligible for both abrasive particles and for solid lubricants. At the higher loads used (approximate to 475-575 Pa) the expected changes in mu were starting to occur for both types of solid particles.